sshproxy: share the caller's LogPolicy.

Now new_connection() takes an optional LogPolicy * argument, and
passes it on to the SshProxy setup. This means that SshProxy's
implementation of the LogPolicy trait can answer queries like
askappend() and logging_error() by passing them on to the same
LogPolicy used by the main backend.

Not all callers of new_connection have a LogPolicy, so we still have
to fall back to the previous conservative default behaviour if
SshProxy doesn't have a LogPolicy it can ask.

The main backend implementations didn't _quite_ have access to a
LogPolicy already, but they do have a LogContext, which has a
LogPolicy vtable pointer inside it; so I've added a query function
log_get_policy() which allows them to extract that pointer to pass to
new_connection.

This is the first step of fixing the non-interactivity limitations of
SshProxy. But it's also the easiest step: the next ones will be more
involved.

nosshproxy.c

@@ -10,7 +10,8 @@ const bool ssh_proxy_supported = false;
 Socket *sshproxy_new_connection(SockAddr *addr, const char *hostname,
                                 int port, bool privport,
                                 bool oobinline, bool nodelay, bool keepalive,
-                                Plug *plug, Conf *conf)
+                                Plug *plug, Conf *conf,
+                                LogPolicy *clientlp)
 {
     return NULL;
 }